Incompatible does not mean "arsehole". Just because I could never be friends or more with a metamour, it doesn't mean that I have to make our partner's life miserable trying to balance the needs of his partner(s). I just have to acknowledge that my metamour's feelings are valid and important and consider that he has obligations to meet their needs as well as mine and compromise accordingly. Whilst there may be times when my needs clash with theirs, I have to accept that my partner will prioritise those needs as he sees fit and if I feel that the way he prioritises those needs results in my needs often going unmet, than that is my job to terminate a romantic relationship that is no longer benefiting me.
We do, however, have to be compatible with him; this means that we have to make sure his existing obligations allow the resources that we require to get what we need out of the relationship we have with him. This might mean that you have an opposing (but ultimately complimentary) schedule to your metamour which offers him the opportunity to spend adequate time with both of us. That isn't about your compatibility with your metamour, in my book, it is still being compatible with your partner.